The invention relates to an electromechanical positioning unit according to the generic term in claim 1. It particularly concerns a positioning unit formed as an inertial driving mechanism with at least one tube-shaped piezoelectric actuator with a movable slider. The actuator has electrodes for connecting it to variable voltages for electrically controlling its motion. The slider is to be moved inside this piezo actuator in the direction of the piezo movements along a tube-shaped sliding surface.
Positioning units of this kind with nanometer precision, activated by piezo actuators based on the principle of inertial driving, are used as positioners with atomic resolution, analysis tools (e.g. scanning tunneling microscope) and tools in micro- and nanotechnology. In these inertial drives the displacement of a slider relative to a sliding surface is achieved by high acceleration of the sliding surface so that the inertial force of the slider exceeds the frictional force and the slider can not follow the motion of the sliding surface.
Especially compact forms of the positioning unit are the one-axis inertial drives as described in DE 38 22 504 Al, in WO-Al-94/06160 and in Review Sci. Instr. 63 (1) (January 1992, pages 263/264). According to these articles one or several piezo actuators move a sliding surface fast and slowly. This particularly tube-shaped sliding surface symmetrically surrounds the slider.
In DE 38 22 504 Al a micromanipulator with several sliders supporting and moving an object is described. The sliders with support heads directly supporting the object are to be moved piezo-electrically inside a tube-shaped sliding surface. However, the frictional force of these sliders having the shape of a cylinder-bolt and not having any flexible elements are not adjustable.
The slider of the positioning unit described in WO-Al-94/06160 ("low-mass support means") is wire-shaped and its frictional force is adjustable by its flexible bendable shape but limited to small variations of the bending. That means only a small bending range is allowed to create a reasonable frictional force. This form of the slider allows inertial driving only if the frictional force is very small. Thus a tip, e.g. the tip of a scanning tunneling microscope, can be moved but not heavy objects. The slider can only exert extremely small forces because these forces are limited to the strength of the frictional force.
Furthermore the forces that can be exerted to the slider in WO-Al-94/06160 are limited by the power capacity of the piezo actuator. The transition from adhesion to sliding strongly depends on the shape of the impulse transmitted from at least one of the piezo actuators to the sliding surface and depends on the mass of the slider. It is also depending on the coefficients of friction of the different materials involved. The piezo actuators depolarize if the applied voltages exceed say a few 100 Volts and after that can not move any more thus limiting the transmission of power into the inertial drive.
In "Review Sci. Instr." 63 (1), January 1992, pages 263/264, a slider made of two halves of a cylinder cut along its axis is described, both cylinder halves hold inside a tube-shaped piezo actuator by frictional forces. For this purpose a spring is introduced between the two halves protecting them from falling out of the tube but allowing the two halves to be moved by piezo forces along the axis of the tube. From this it is known that it is extremely difficult to adjust the spring mechanically so that the two halves of the slider can be moved by piezo forces along the axis of the tube. The mechanical construction of the slider with the two halves of the cylinder and the spring, that is difficult to adjust, is so complicated that the diameter of the piezo actuator can not be less than 19 min to 20 mm practically. The bulkiness of the whole construction makes it sensitive for mechanical disturbance from outside like acoustic waves from walking steps making the positioning with atomic precision impossible normally without special vibration isolation systems.
The practical application of a positioning unit affords that the drive is tolerant against variations of the frictional forces arising from aging, dust, corrosion of the slider, change of the spring constant etc. This tolerance is not given by the unit in Review Sci. Instr. 63 (1). In this article the motion of the slider against gravity could only be achieved by driving the "Piezoelectric ceramics" with voltages exceeding 50 volts. Operation against forces larger than gravity is not possible; particularly the unit can lift nothing by itself except the slider.
According to the above mentioned Review Sci. Instrum. 63 (1) the slider-mass consists of two cylinder halves that are pressed against the friction tube wall with the help of a spring. If during operation both mass parts have different friction against the friction tube wall, which can hardly be avoided in "micro"-practice, both mass parts run asynchronous. By this they can tilt and stick in the friction tube. So a substantial disadvantage of the known device is, that the slider always must consist of two cylinder halves and therefor of two mass units, that are pressed transversally to the transport direction against the inner wall of the friction tube.